1. Field of the Invention
The present invention relates to a frequency modulator which generates an image clock used for ON/OFF control of a laser beam scanning the surface of an image carrier such as a photosensitive drum.
2. Description of the Related Art
In an electrophotographic image forming device, it is conventional to scan an entire document placed on a table by means of a longitudinal fluorescent member and an integrally formed mirror in longitudinal and vertical sub-scanning directions. The beam emitted from the fluorescent member is reflected to the original, is reflected by an integral mirror and other installed mirrors, and enters into a photoelectric conversion sensor. A digital image is obtained through AD conversion. Through decoding of the received data in a facsimile machine, and through expansion processing of the code in a printer, the resultant digital images are stored in a hard disk or a memory. The digitized image data undergoes image processing according to desired output properties, and then modulated and converted again by a laser drive into light.
FIG. 10 schematically illustrates an optical system for output. A laser device 1003 emits a laser beam onto a polygon mirror 1001. The polygon mirror 1001, rotationally driven by a motor (not shown), reflects the laser beam onto a photosensitive drum 1002 so as to scan the drum 1002 along a straight line. The components of the optical system are arranged in a three-dimensional arrangement such that only the laser beam reflected off the polygon mirror 1001 passes through an fθ lens 1005.
When the polygon mirror 1001 rotates at a constant speed, the scanning rate at the surface of the drum 1002 near the center of the polygon mirror 1001 tends to be slower than at the longitudinal ends of the drum 1002, which is more distant from the center of the polygon mirror 1001. However, the fθ lens 1005 cancels this tendency via refraction of light, thus ensuring that the laser beam passes over the surface of the drum 1002 at a constant speed. In FIG. 4, the solid line represents the laser beam speed characteristics, and the dotted line represents an ideal curve after correction. In general, however, it is difficult to achieve such ideal lens properties because of cost restrictions and the like. The resulting error components are a factor causing deterioration of the image quality in the form of distortion and color shifting in a color electrophotographic device. Each face of the polygon mirror 1001 corresponds to one scanning line.
Referring back to FIG. 10, a laser detector 1004, arranged by the side of the photosensitive drum 1002, detects the reference position of the laser, and serves as a sensor for determining an irradiating period of image information from the laser device 1003. The photosensitive drum 1002 forms a latent image as a surface potential by receiving laser irradiation while rotating. Then, a toner image is formed on a medium such as paper, through adhesion and transfer of toner. After fixing the toner onto the medium, the toner image is discharged.
A method for replacing the fθ lens or improving the image quality by intensifying properties of the fθ lens by causing a gradual change in clock frequency for modulating the image signal has been devised. The pixel pitch in the absence of an fθ lens is wider at the drum center and narrower at the ends, as shown in FIG. 9, under the effect of the laser scanning rate shown in FIG. 4. A higher modulation frequency leads to a narrower pixel pitch after forming the latent image, and a lower modulation frequency results in a wider pixel pitch, as shown in FIG. 8.
By use of this relationship, unevenness of the pixel pitch caused by the absence of an fθ lens can be corrected by controlling the modulation frequency. Similarly, even in the presence of an fθ lens, a correction error can be corrected.
When realizing the above-mentioned means by hardware circuits, it is not desirable, from cost considerations, to have a memory for one line for storing the correction curve.
A conceivable solution to this problem is to provide an IC for laser output based on a method of making a correction using an approximate curve by dividing a line into segments comprising a plurality of pixels. The method comprises the steps of counting pixels scanned in one line, generating timings for segment boundaries, storing in advance only modulation frequencies at ends of the segments, calculating the intervals by linear interpolation, and outputting the modulation frequencies. Another conceivable method is to store the modulation frequency for each segment, and to make a correction to a certain value within each segment. In FIGS. 5 and 6, the abscissa represents the position on the photosensitive drum. The main scanning lines are divided into segments A to G, and a correction as shown by a solid line is conducted relative to the ideal curve represented by the dotted line.
(For example, Japanese Patent Laid-Open No. 05-203889).
In a correction made in unit segments, however, correction errors tend to occur, and at segment boundaries, changes in the amount of error are discontinuous, that is, not smooth. FIG. 7 illustrates errors occurring when making a correction with a constant value within each segment. As shown, steps are observed between the amounts of error at segment boundaries. Since the pixel pitch exerts a visual influence, such as the occurrence of moiré fringes, frequency distribution, or color shifting, the generation condition of moiré fringes or color shifting may change suddenly at segment boundaries.
Furthermore, the above-mentioned control has a problem in that, the segment boundaries, which line up along a straight line in a direction perpendicular to the main scanning direction of the laser (hereinafter referred to as the “sub-scanning direction”), are visually easily detectable.
For the purpose of avoiding occurrence of a similar phenomenon, Japanese Patent Laid-Open No. 9-218370 proposes a method of performing control so that positions of correction pixels do not form a line in the sub-scanning direction. In the segment processing method of the present application which conducts correction for all pixels in the lines, practical application is impossible.